1. Field Of The Invention
This invention relates to the field of electronic filters and amplifiers for electroacoustic systems such as hearing aids, and more particularly to methods and devices for correction and clinical testing of hearing impairment.
2. Description Of The Related Art
The need for improved hearing aids and audiological fitting procedures is widely attested to by research efforts worldwide. It has been said that over 28 million Americans have hearing impairments severe enough to cause a communications handicap. While hearing aids are the best treatment for most of these people, only about 5 million actually own hearing aids, and fewer than 2 millions are sold annually. In addition, less than 60% of hearing aid owners are actually satisfied with their hearing aids.
Hearing impairment is most commonly expressed as a loss of sensitivity to weak sounds, while intense sounds can be as loud and uncomfortable as in normal hearing. State-of-the-art hearing aids treat this phenomenon of “loudness recruitment” (or loss of dynamic range) with sound amplification that automatically decreases with sound amplitude. This technique, known as “wide dynamic range compression” (WDRC), compresses the range of normally experienced sound amplitudes to the smaller range required by the impaired ear. Loudness recruitment is the basic audiological problem addressed by modern hearing aids.
Broad agreement exists that the most general and potentially successful design is a multi-channel compressive hearing aid that addresses the compression needs of each band of audible frequencies. Sharp disagreement exists, however, whether the dynamic range compression should be rapid or slowly adapting (Villchur, E., Signal processing to improve speech intelligibility in perceptive deafness, J. Acoust. Soc. Am. 53, 1646-1657 (1973); Plomp, R., The negative effect of amplitude compression in multichannel hearing aids in the light of the modulation-transfer function, J. Acoust. Soc. Am. 83, 2322-2327 (1988); Plomp, R., Noise, amplification, and compression: consideration of three main issues in hearing aid design, Ear and Hearing 15, 2-21 (1994).
Thus, the best engineering approach to compression has been uncertain. Rapid compression amplifiers protect the ear from uncomfortable changes in loudness, but nonlinearly distort the sound waveform. Slowly adapting compression avoids distortion, but allows some loudness discomfort. Resolving these competing interests have plagued previous efforts to develop suitable hearing aids employing wide dynamic range compression (WDRC).
Recent advances in hearing aid development have been largely driven by availability of inexpensive miniaturized electronic analog and digital signal processors. The classical audiological problem of loudness recruitment, which older hearing aids solved with a manual volume control, is now solved with sound compression systems that automatically provide greater amplification for weak than for intense sounds. A recent comprehensive and authoritative review found that (1) “for speech in quiet at a comfortable level, no compression system yet tested offers better intelligibility than individually selected linear amplification” (i.e., manual volume control), and (2) “in broadband noise, only one system, containing wideband compression followed by fast acting high-frequency compression, has so far been shown to provide significant intelligibility advantages.” (See Dillon, H., Compression? Yes, but for low or high frequencies, for low or high intensities, and with what response times?, Ear and Hearing 17, 287-307 (1996) [comments by Villchur, and reply by Dillon, 1997, in Ear and Hearing, 18:169-1731]).
The technology that has dominated public hearing aid research is linear amplification with sound level dependent gains that are adjusted either manually or automatically to provide the desired wide dynamic range compression (WDRC). The use of linear amplifiers has been the dominant compression technology for hearing aids, be they analog or digital, single or multiple channel (see Levitt, H., Pickett, J. M., and Houde, R. A., Sensory Aids for the Hearing Impaired, IEEE Press, NY. (1980); Goldstein, J. L., Valente, M., Chamberlain, R., Acoustic and psychoacoustic benefits of adaptive compression thresholds in hearing aid amplifiers that mimic cochlear function, J. Acoust. Soc. Am. vol. 109, p. 2355 (2001)).
Villchur's above-cited 1973 article proposes the use of adaptive linear compression to reduce the dynamic range of the fine structure of speech signals with greater amplification of weak than strong syllables. To achieve this result, the adaptive linear compression system disclosed by Villchur must use short release times. However, the use of short release times is less than desirable, because it causes excessive amplification of unwanted ambient sounds during normal pauses in speech.
Dillon's above-cited article also reviews the use of linear amplifiers to implement a WDRC system. In these systems, the “compression threshold” is the input sound level above which the gain of the linear amplifier is adapted to reduced linear gains.
An innovative design by Engebretson and Morley (See Engebretson, A. M., Morley, R. E., and Popelka, G. R., Development of an ear-level digital hearing aid and computer assisted fitting procedure, J. Rehab. Res. Devel., 24 (4), 55-64 (1987); U.S. Pat. No. 5,357,251 issued to Morley et al.) was an adaptive linear WDRC digital amplifier with four channels partitioning the audio frequency range of 375 Hz-6000 Hz into four octave bands. In this design, each channel is configured to provide maximum corrective gain for low amplitude signals. The corrective gain is reduced at larger amplitudes by adaptive linear amplification. The BPNL transducers are linear with symmetrical hard limiting, i.e., T(x)=−T(−x) (defined as “odd symmetry”), which prevents even-order harmonics and intermodulation tones from being generated by limiting. The second filter in each channel reduces the odd-order distortion that is caused by the limiting. Considerable engineering sophistication was applied to the implementation of this design into a programmable, in-the-ear, practical digital hearing aid.
In common with other compressive hearing aids, the Engebretson and Morley design implements adaptive linear WDRC amplification of sounds using linear amplifiers. However, the normal cochlea employs essentially non-linear compressive sound amplification, which is degraded by sensorineural impairment to a linear residual response. Basic cochlear research has generated a rich body of experimental data on non-linear phenomenology whose salient features and interrelations have been described with mathematical models. (See Goldstein, J. L., Modeling rapid compression on the basilar membrane as multiple-bandpass nonlinearity filtering, Hear. Res. 49, 39-60 (1990); Goldstein, J. L., Exploring new principles of cochlear operation: bandpass filtering by the organ of Corti and additive amplification by the basilar membrane, In Duifhuis, H., Horst, J. W., van Dijk, P. and van Netten, S. M., Eds. Biophysics of Hair Cell Sensory Mechanisms. World Scientific, Singapore, pp. 315-322 (1993); Goldstein, J. L., Relations among compression, suppression, and combination tones in mechanical responses of the basilar membrane: data and MBPNL model, Hear. Res. 89, 52-68 (1995). The inventor herein has determined from these models that there is a need to depart from the conventional design implementing WDRC amplification with linear amplifiers.
The parent application (U.S. patent application Ser. No. 09/158,411 filed Sep. 22, 1998, the entire disclosure of which is incorporated by reference) discloses how the models may be used to: (1) specify the shape of quiescent compression characteristic to approximately restore the normal cochlear best frequency response; (2) implement compression rapidly with instantaneously responding, memoryless compressive transducers derived from cochlear models; and (3) enhance the properties of instantaneous compression by adopting the cochlear strategy of non-linearly mixing linear and compressive responses. The parent invention improved on the Engebretson and Morley design by employing at least one variable gain channel comprising a linear transmission path of constant gain, a compressive transmission path of higher gain than the linear transmission path, and a non-linear adder combining the outputs of the linear in the compressive transmission paths, wherein the variable gain channel is configured to provide relatively higher gain at low levels, rapid gain compression at intermediate levels converging to linear gain at high signal levels, and slow AGC control of the compressive gain.
The invention disclosed in the parent application, among other things, provides two types of enhancements over conventional linear WDRC models: (1) restoration of waveform modulation lost in rapid compression, and (2) reduction in amplification of unwanted background noise in the presence of more intense desired signals.
In subsequent research, it was discovered by the inventor herein that both enhancement goals can be achieved by adapting the compression thresholds of the memoryless compressive non-linear transducers. (See Goldstein, J. L., Valente, M., Chamberlain, R., Gilchrist, P., and Ivanovich, D., Pilot experiments with a simulated hearing aid based on models of cochlear compression, IHCON 2000, Lake Tahoe, Calif. (2000); and the above-cited 2001 article by J. Goldstein) This adaptation is functionally similar to modifications in the normal cochlear response produced by “tail suppression” (for a fuller understanding of “tail suppression”, see Kiang, N. Y. S. and Moxon, E. C., Tails of tuning curves of auditory-nerve fibers, J. Acoust. Soc. Am. 55, 620-630 (1974); Abbas, P. J. and Sachs, M. B., Two-tone suppression in auditory-nerve fibers: Extension of stimulus response relationship, J. Acoust. Soc. Am. 59, 112-122 (1976); Duifhuis, H., Level effects in psychophysical two-tone suppression, J. Acoust. Soc. Am. 67, 914-927 (1980); Ruggero, M. A., Robles, L. and Rich, N. C., Two-tone suppression in the basilar membrane of the cochlea: Mechanical basis of auditory-nerve rate suppression, J. Neurophys. 68, 1087-1099 (1992); and efferent mechanical control (for a fuller understanding of efferent mechanical control, see Mountain, D. C., Changes in endolymphatic potential and crossed olivocochlear stimulation alter cochlear mechanics, Science 210, 71-72 (1980); Gifford, M. L., and Guinan, J. J., Effects of crossed-olivocochlear-bundle stimulation on cat auditory nerve fiber responses to tones, J. Acoust. Soc. Am. 74, 115-123 (1983); Murugasu, E., and Russell, I. J., The effect of efferent stimulation on basilar membrane displacement in the basal turn of the guinea pig cochlea, J. Neurosci. 16, 325-332 (1996).
Studies by the inventor have shown that when processing clean speech (speech in a relatively quiet environment having little or no unwanted background noise), the compression threshold can be maintained at a predetermined quiescent level with the result being little or no degradation in sound quality. This result generally holds true when the compression threshold is between a range of the predetermined quiescent level and about 20 decibels below the average sound level of the received sound signal.
However, when that same speech is processed by the hearing amplification device in a relatively noisy environment, the sound quality of the amplified sound signal (now containing the speech plus background noise) resulting from the static predetermined quiescent compression threshold is less than optimal due to overamplification of the background noise. In other words, the signal-to-noise ratio (SNR) of the sound signal is degraded by the amplifier.
The inventor herein has found that by adjusting the compression threshold from its quiescent level to a range between about 5 decibels below and about 5 decibels above the average sound level of the received sound signal, overamplification of unwanted background noise in the sound signal can be reduced while still maintaining appropriate amplification of the desired speech.
Therefore, by adapting the compression threshold of the linear-to-compressive gain characteristic, the present invention provides an elegantly simple implementation for enhancing rapid and instantaneous compressive amplification that mimics useful cochlear function while avoiding its complex structure.